The recent expansion of telecommunications and computer communications, especially in the area of the Internet, has created a dramatic increase in the volume of worldwide data traffic that has placed an increasing demand for communication networks providing increased bandwidth. To meet this demand, fiber-optic networks and dense wavelength-division multiplexing (DWDM) communication systems have been developed to provide high-capacity transmission of multi-carrier signals over a single optical fiber. In accordance with the DWDM technology, a plurality of superimposed concurrent optical signals is transmitted on a single fiber, each signal having a different central wavelength. In DWDM optical networks, optical transmitters and optical receivers are tuned to transmit and receive on a specific wavelength.
With the widespread deployment of DWDM optical networks, knowing precisely what is happening at the optical layer of the network is quickly becoming a real-time issue for network management. Stable and protected DWDM links cannot be realized without real-time optical monitoring at each channel. For example, as the number of channels deployed in a DWDM optical network increases, say from 40 to 80 or 160, wavelength drifts and power variations are more likely to cause data errors or transmission failures. It is therefore becoming important for network management to dynamically monitor the performance of the communication channels in order to supply the corresponding decision-support systems with information necessary for fault detection and identification, as well as for undertaking efficient restoration actions. To achieve this goal, a new type of fiber-optic products has been developed, the so-called optical performance monitors (OPM).
An OPM consists of a spectrometric transducer and a computing means. The spectrometric transducer converts an optical signal into a digital signal representative of the spectrum of the input optical signal. The computing means processes that digital signal in such a way as to provide the final result of measurement, i.e. an estimate of the parameters of the optical signal being monitored in a DWDM system. A fractional portion of optical power, typically 2%, is tapped from the mainstream optical signal running through the optical fibre, using a tap coupler. The purpose of tapping is monitoring the optical signal while keeping the properties of the main traffic unchanged. Since the tapped signal will not be added back to the mainstream signal, there is little effect on the properties of the transmitted data, and the OPM thus provides an almost non-invasive measurement. The weak signal tapped from the mainstream optical signal is then directed to an optical unit, by which the channelized wavelength components are separated. The optical unit therefore performs a spectral decomposition of the optical signal; the results of that decomposition are detected by a detector. The detector converts optical signals into electrical signals. The electrical signals are transmitted to the electronics circuitry for processing and digital output.
The principle of operation of a spectrometric transducer may refer to various physical phenomena that make possible separation of spectral components of the input optical signal. The following devices are examples of technical means used for this purpose:                a grating or a linear variable filter that enables separation of spectral components in space;        a tunable filter that enables separation of spectral components in time;        an optical heterodyne that enables shifting the spectrum in a wavelength range where its analysis is easier.        
A conventional optical network performance monitor typically contains a detection element that is responsive to the combination of all signal channels carried by a main signal stream, and that is operative to generate data containing information of a collective power level provided by all channels. Such data generated in the electrical domain are not sufficient to provide detailed information of channel performance. For instance, if a power level of one of a plurality of channels of the mainstream signal is decreased while a power level of another channel is increased, a total power level measured by such device typically remains constant, thereby providing an inaccurate indication of a monitored network performance. Thus, in order to monitor a condition of an individual channel in a DWDM network, performance monitoring is preferably carried out in the optical layer. An OPM constitutes an integrated spectrometric device at a module level operating in the optical layer, the device which is capable of monitoring the performance of all individual channels, and of providing rapid channel identification, i.e. the estimates of power (P), central wavelength (λ), and optical signal-to-noise ratio (OSNR) for each channel. In order to respond to higher channel counts and transmission speed, the efforts of improving the performance of OPM have been often focused on enhancing the performance of the optical part of the spectrometric transducer which in turn resulted in a high design complexity and high manufacturing risks.
Following an RHK report (“Vendors Must Adapt Products, Strategies to Stake a Claim in Crowded OPM Market”, Insight, January 2002), OPMs may be classified into three groups: Type-I OPMs, Type-II OPMS, and Type-III OPMs. A Type-I OPM is a monitor capable of providing real-time measurements of power for each DWDM channel. A Type-II OPM is a monitor capable of providing real-time measurements of power, central wavelength, and optical signal-to-noise ratio for each channel. A Type-III OPM is able, moreover, to predict indicators of the service quality provided by a DWDM system such as the bit-error rate (BER) and Q-factor (Q). Currently, those indicators can be correctly measured only with out-of-service test equipment, using a known test sequence in place of the real signal. The determination of BER and Q therefore takes place in the electrical domain, after a signal received by the DWDM receiver is passed on to the electronic circuit. Obviously, this is an expensive, time-consuming and cumbersome method.
The Type-I OPM commonly uses demultiplexing-type spectrometric transducers. Since a demultiplexing-type component, e.g. an AWG, gives a set of fixed discrete channels with a pre-defined frequency interval, i.e. channel spacing, such OPM is only able to provide power measurements at the wavelength positions corresponding to the DWDM channels. It is obvious that the measurements will be biased when there is thermal drift of some wavelength-related properties of the optical part of the OPM. A type-II OPM is able to provide more network information than a type-I OPM since it not only measures power, but also wavelength variation and OSNR.
It is not only P, λ, and OSNR of each channel that is of interest of in-service monitoring of a DWDM system, but also the bit error rate, BER, and Q, an indicator of overall quality of the signal. The conventional approach in determining BER makes use of out-of-service test equipment, and is time consuming and expensive. An obvious approach to in-service BER or Q reporting is an electrical-domain approach. It consists of tapping off a part of an optical signal, demultiplexing it through a tunable filter, detecting and then electrically regenerating it through a DWDM-type receiver. However, this approach presents various drawbacks. It is an expensive and time-consuming method since it operates in a serial manner—channel by channel—using serial channel scanning and BER or Q processing. Further, BER or Q is influenced by the receiver noise. Also, the tunable filter introduces signal distortions contributing to an increase in BER (decrease in Q) or a reduction in reliability.
Typically, in conventional applications, BER is determined by counting bits, a process which takes place in the time domain. Assuming a regular BER value in the order of 10−12, and assuming a transmission bit rate of 2.5 Gb/s, it is to be expected that—on average—every 400 s of data flow one faulty bit is to be detected. To determine BER estimate with an acceptable accuracy, a testing time of several hours is necessary.
It would be highly advantageous to have at one's disposal alternative, and simpler monitoring methods for faster fault detection and localization. Optical-domain methods, even if less accurate than electrical ones, can provide a fast, a simple, and an economical approach to reach this goal.
It would be of further advantage to have at hand a system that allows for real-time estimation of OSNR and BER or Q from the data representative of a real-world signal without the need for specific test signal sequences.